1. Field of the Invention
The present invention relates to an improvement in exhaust gas sensors utilizing a change in resistance value of metal oxide semiconductors, and more particularly to an improvement in durability of electrodes. The exhaust gas sensor of the invention is suitable for detection of the air-fuel ratio in automobile engines, space heaters or boilers, etc.
2. Prior Art
Hitherto, a problem that Pt electrodes of an exhaust gas sensor are corroded by high temperature reducing atmosphere has been well known. For example, U.S. Pat. No. 4,237,722 proposes the use of a Pt-Rh alloy as highly durable electrodes, disclosing that such corrosion is caused by reaction of Pt with carbon in the exhaust gases.
The inventors' tests confirmed that the Pt-Rh alloy had excellent durability, when TiO.sub.2 was used as a metal oxide semiconductor (Table 2), but the situation is quite different with compounds containing an Sn compound such as SnO.sub.2, BaSnO.sub.3, etc. When the Pt-Rh alloy electrodes are used with these compounds, the electrodes are corroded in high temperature reducing atmospheres. The Pt-Rh alloy electrodes are effective with TiO.sub.2, but are ineffective with BaSnO.sub.3 or SnO.sub.2, etc. Elementary analysis of corroded electrodes revealed the formation of solid solutions of Pt and Sn. The cause of corrosion is not the reaction of Pt with carbon, but the formation of an alloy of Pt and Sn.
The inventors further found that Pt-Rh electrodes were corroded by metal oxide semiconductors, including Fe, Ni, or Co elements, because of the alloy formation with Pt. When these elements are used, the electrodes are corroded in a high temperature reducing atmosphere. Main materials of exhaust gas sensors containing these elements include such perovskite compounds as SrFeO.sub.3, LaNiO.sub.3 and LaCoO.sub.3, etc.
According to related prior arts, TiO.sub.2 is a representative exhaust gas sensor material and its characteristics are described in U.S. Pat. No. 3,886,785, etc. SrFeO.sub.3, LaNiO.sub.3 and LaCoO.sub.3 are p-type metal oxide semiconductors and their resistance values diminish with oxygen concentrations and increase with combustible gas concentrations. These compounds all have crystalline structure of perovskite type, and characteristics of SrFeO.sub.3 are described, for example, in U.S. Pat. No. 3,558,280, while those of LaCoO.sub.3 and LaNiO.sub.3, for example, in "Proceedings of the International Meeting on Chemical Sensors" (Kodansha 1983) and U.S. Pat. No. 4,507,643, etc. SnO.sub.2 is an n-type metal oxide semiconductor, and its characteristics are well-known as shown in U.S. Pat. No. 4,459,577, etc. Metal oxide semiconductors such as BaSnO.sub.3, CaSnO.sub.3 and SrSnO.sub.3, etc., are perovskite compounds and novel materials for exhaust gas sensors. Characteristics of these compounds are described in U.S. patent application No. 807,257 (Dec. 10, 1985) filed by the same assignee, and its descriptions will be cited hereinafter.